4WD lights on the dash were blinking on and off?

Ford explorer, 1998, 77671 miles. When I started the car and went for a drive the (4wd-high and 4wd-low) lights on the dash were blinking on and off. They would blink about 5 time then turn off then repeat blinking again after a few mins. When I switch from 4wd-auto to high and also to low there was no change both times the dash lights did not light to show which mode I was in. Could there be a problem with the 4wd? Have you heard of this problem? Thank you.